A read speed for a NAND flash memory is conventionally controlled mainly by a recovery time for bit lines. The recovery time is a time required to recover the potential of a bit line to an original value when the potential is disturbed. The recovery time depends on the capacitive coupling between the bit line and an adjacent bit line or a second adjacent bit line, or the charging capability of the bit line. Increasing miniaturization has led to a decrease in cell current and an increase in mutual interference between the bit line and the adjacent bit line or second adjacent bit line. This has caused the read characteristics of NAND flash memories to be further degraded.
Furthermore, read operations include writes generally performed as internal operations of a NAND flash memory or threshold checks (hereinafter referred to as verifications) carried out on memory cells during erase operations. Thus, not only the read characteristics but also the write or erase characteristics of the NAND flash memory tend to be degraded.